1. Field of the Invention
The present invention relates to an image capturing system, signal processing circuit, and signal processing method.
2. Description of the Related Art
Many image capturing systems such as a digital camera, video camera, copying machine, or a facsimile apparatus incorporate an image sensing apparatus in which pixels each including a photoelectric conversion element are arrayed one- or two-dimensionally. The image sensing apparatuses include a CCD image sensing apparatus or an amplifying image sensing apparatus having amplifying elements in the pixel region.
Recent image sensing apparatuses tend to increase the number of pixels. As the area of one pixel decreases, the area of a photoelectric conversion element also tends to decrease. That a photoelectric conversion element has a smaller area indicates that an amount of incident light per pixel becomes small. In, for example, an amplifying image sensing apparatus having a plurality of elements in one pixel, the area occupied by the photoelectric conversion element in each pixel is small. Hence, the incident light amount tends to be smaller.
To solve this problem, an image sensing apparatus in which a plurality of adjacent pixels (photoelectric conversion elements) share an electrical function has been proposed (Japanese Patent Laid-Open No. 10-256521). In this case, the element layout relationship near the photoelectric conversion element may change depending on the adjacent pixel.
An image sensing apparatus sometimes has defective pixels. A defective pixel outputs a signal (abnormal signal) that is largely different from a normal signal. Following methods of correcting such an abnormal signal have been proposed.
Japanese Patent Laid-Open No. 2001-045382 has proposed a defective pixel correction method for an image sensing apparatus in which a plurality of photoelectric conversion elements share an amplifying element. As shown in FIG. 17, four photodiodes a11, a12, a21, and a22 each functioning as a photoelectric conversion element share an amplifying element MSF. If one of the photodiodes is defective, the signal from the defective pixel is interpolated and then added.
An image sensing apparatus sometimes generates vertical stripe-shaped noise depending on its read circuit. A technique of solving this problem is known, which detects a fixed pattern noise component that reflects variations in the row direction of a read transistor by using a non-effective pixel having no photodiode as a dummy line, thereby canceling the fixed pattern noise component (Japanese Patent Laid-Open No. 2005-176061).
In the arrangement of Japanese Patent Laid-Open No. 10-256521, if a defective pixel signal is corrected by simply using the signals of neighboring pixels, as in general practice, the dark current difference may affect the result.
This is because the dark current affects the signals in different manners depending on the difference in the pixel structure. It is impossible to appropriately correct a signal when the influence of the dark current is large.
In the method disclosed in Japanese Patent Laid-Open No. 2001-045382 as well, a signal is interpolated using neighboring pixels and then added. In signal interpolation, however, the variations in the dark current caused by the difference in the pixel structure are not taken into consideration.
Particularly in signal correction in an optical black region (OB region) for outputting a black reference signal, the influence of the variations in the dark current is larger than in signal correction in an effective area for forming an image signal.
Japanese Patent Laid-Open No. 2005-176061 discloses a technique of canceling vertical stripe-shaped fixed pattern noise. However, this technique does not consider fixed pattern noise generated by pixel structures in an image sensing apparatus including a plurality of pixels having different pixel structures.